1. Field of the Invention
This invention relates to a novel method and apparatus for monitoring the temperature of the gases exiting from a combustion chamber and particularly to a method and apparatus for use in monitoring the operation of gas turbines.
2. Description of the Prior Art
In a gas turbine, a compressor supplies air for combustion to a plenum, to which one or more combustion chambers are connected. The chambers are mounted and supply hot gas at their outlets to impact vanes on the turbine wheels. Combustion failure, or non-uniform combustion, in one or more combustion chambers not only reduces the turbine output, but also results in thermal stresses in the turbine wheels and uneven impulsion of the wheels. Thermal stress and uneven impulsion reduce the life of the turbine wheels, so that rapid, automatic shut-down of the turbine is necessary to prevent damage to the engine if a combustion failure occurs.
It is well known to monitor combustion chamber operation by optical observation using ultraviolet sensors, for example. But this suffers from the defect that shut-downs can occur by reason of false indications of failure, due to soot accumulation on the observing window. In any event, this process, known as "flame proving", simply indicates that combustion is present, and gives no indication of gas temperature.
A further problem occurs in gas turbine engines with multiple combustion chambers. If the fuel flow to each combustion chamber is not the same, the gas temperatures and ve1ocities entering the turbine will be different at various circumferential locations. This subjects the turbine buckets to both mechanical and thermal vibration. Typical stationary gas turbine engines may tolerate only 50.degree. F. circumferential variation in the temperature of the gas leaving the turbine wheel.
Then too, absolute temperature of the gas entering the inlet nozzles to a turbine wheel is critical. Hence, it is necessary to monitor not only the presence of combustion, but the temperature of the combustion products. High thermal efficiency demands operation with as high a gas temperature as possible. But the strength of the materials from which the nozzles and turbine buckets are made puts an upper limit on operating temperature. Thus, for high efficiency the engine must operate close to the upper temperature limit and with a minimum safety factor in the turbine inlet gas temperature. Because of the very harsh conditions at the inlet to the turbine nozzles, it has not been feasible to use thermocouples or other temperature sensors to monitor the turbine inlet temperature directly. Temperature sensors may be placed in the exhaust from the turbine wheel, but this requires that the inlet temperature be calculated from the outlet temperature and the load on the turbine. Changes in load, air flow, or fuel flow can produce rapid changes in gas temperature at the turbine inlet. An object of this invention is the provision of a way to quickly detect the temperature and any changes in the temperature of a gas used to drive a high-performance turbine.